Why did the Western Roman Empire collapse? The question has consumed historians, clergy and philosophers since its fall in the 5th century. Today, the generally accepted answer is that the empire’s demise (the Eastern Roman —or Byzantine—Empire lingered until 1453) stemmed from multiple causes that, in combination, pushed the empire created by Augustus into irrevocable decline.

But more meaningful questions concern Rome’s prior longevity and dominance. What strategic and military factors fostered the empire’s success for more than two centuries? And what strategic and military changes prefaced its downfall?

Among the difficulties facing classical historians is that primary sources virtually dry up in the 3rd century, when a great crisis broke on the Romans—one that lasted nearly 50 years, with the empire besieged from abroad and embroiled in a series of civil wars. When historical sources reemerge at the end of that century, they suggest a radically different military, political and cultural epoch. Even extant 1st and 2nd century sources present difficulties, offering little more than an outline of overall military and strategic policies. And the authors on whom we must rely (e.g., Suetonius or Tacitus) were not interested in the questions that engage military historians today.

Finally, the vocabulary used by ancient historians is not congruent with that used by 21st century strategic analysts. One result is a misreading of the past by those modern-day historians who argue the Romans had no concept of grand strategy. But the continuities of the period between Augustus’ creation of the principate in 27 BC and the onset of the troubles in AD 235 do clearly suggest the existence of a coherent grand strategy over some 250 years. That strategy carefully measured means and ends, balanced diplomacy and military force and determined what was—and was not—feasible and defensible when adding territory to the empire.

Any coherent grand strategy demands the setting of clear and attainable goals based on an accurate estimate of the means a state possesses and can mobilize. When Augustus (then known as Octavian) became sole ruler of the Roman world in 31 BC, the borders of Roman rule claimed little geographic coherence. In the West, the Rhine frontier provided considerable protection, but tribal societies held much of western Spain and most lands south of the Danube. The empire’s frontiers extended over great distances, and in almost every quadrant Rome confronted many serious threats from external enemies.

Simply put, the new political entity Augustus was creating made great political and economic sense but little military and strategic sense. Moreover, his empire was based on a subsistence economy that produced relatively small surpluses to support the civilization for which it is justly famous and the military on which its peaceful existence depended. Among the many political difficulties Augustus faced after defeating his rival Mark Antony at the 31 BC Battle of Actium was how to establish a government in Rome that possessed sufficient legitimacy to prevent the rise of generals and politicians who might attempt to seize power, a threat that had marked the Republic’s catastrophic course over its final half century. Augustus’ answer was to create a sort of disguised monarchy, the principate, which kept the form of the republic but placed extraordinary powers and control of Rome’s army in the hands of one man.

Augustus faced two other major challenges: The first was to downsize the massive military forces he and Antony had inherited from the civil wars and used in their fight for control of the empire. The second concerned the establishment of defensible frontiers for the Roman world. The problems were interrelated, as the army would need sufficient strength to keep the barbarians out of the Mediterranean and would need to mount that defense without breaking the empire economically.

How Augustus met those two challenges suggests a great deal about his grand strategy. On the military side, he reduced the army to an all-professional force of 28 legions that came under his direct control through officers he appointed. In addition, the Romans established a number of auxiliary formations, which comprised about the same number of soldiers. Altogether, Augustus could field approximately 300,000 soldiers to defend the entire Mediterranean region—an astonishingly small number considering the territories and enemies involved.

Rome possessed history’s first true professional army, a tactically proficient force that sustained its excellence through a severe regimen of training and discipline that remained in place into the 3rd century. The Roman army was also proficient at building road networks that buttressed frontier defenses and the empire’s grand strategy by allowing the rapid transfer of units from one distant region to another. The roads also enabled the resupply of the legions and auxiliary cohorts spread across the frontier’s widely separated fortresses and guard posts. Similarly, the legions constructed great defensive lines— such as Hadrian’s Wall in northern Britain—as bases from which they could strike at enemies beyond the Roman equivalent of the Irish “pale.” The engineering sophistication of both roads and fortifications underscore the deep appreciation Roman strategists had for maximizing both the offensive and defensive potential of their military instrument.

Rome’s military superiority over its neighbors lasted well over two centuries. The Romans bested their enemies in battle after battle, thanks to their systematic, coherent approach to training and tactical doctrine. They won because their troops were a disciplined army rather than an armed mob, and when the legions and auxiliaries fought together, the whole was truly greater than the sum of its parts.

With his 28 legions, Augustus waged a series of wars to achieve defensible frontiers. Those wars brought the tribes of northwestern Spain, Switzerland and the Balkans under Roman control. The latter success, though, led to a massive revolt of the Danubian tribes in AD 6, which forced Augustus to engage 10 legions under his stepson Tiberius (who would eventually succeed him as emperor). The strain of the Balkan wars on the empire’s manpower and economic resources was such that Augustus had to draft slaves to keep Tiberius’ legions up to combat strength.

In AD 9 disaster struck in the new province of Germania, when rebels led by Arminius ambushed the Roman governor, Publius Quinctilius Varus, and his three legions in the Teutoburg Forest and slaughtered them almost to the last man. While historians have argued that Varus’ defeat pushed the Roman frontier from the Elbe River back to the Rhine, the Romans had other reasons for pulling back to the Rhine. Given the subsistence nature of their economy and the difficulties involved in transporting the grain and other supplies on which frontier legions depended, it made greater logistic sense to maintain bases along the Rhine, closer to the food-producing regions of Gaul. Moreover, the Germany of AD 9 was simply incapable of providing the foodstuffs a Roman army deployed along the Elbe would have required.

Over the next 250 years the Romans managed their northern frontiers with a combination of diplomacy, manipulation and punishing military force when necessary. Rome’s approach to its frontier troubles is vividly illustrated by the case of Dacia. During the reign of Domitian (81–96), the Dacian leader Decebalus united the tribes in what today is Transylvania. The Romans saw him as a serious threat to their provinces south of the Danube, and the Dacian king proved them correct by invading Moesia and causing considerable damage. The Romans soon drove the Dacians from the south bank of the Danube and by late 88 had defeated Decebalus. The legions were on the brink of besieging his capital when troubles on the Rhine frontier compelled Domitian to make peace and present the Dacians a sizeable subsidy.

Trajan’s (r. 98–117) accession as emperor two years after Domitian’s assassination in 96 quickly led to a reconsideration of Roman policy toward Decebalus. The soldier-emperor brought together forces from throughout the empire and in 101 led four legions across the Danube. The Dacian Wars proved successful but expensive. By the second war, Trajan had concentrated parts or all of nine legions to break the power of the Dacians and create a new province across the Danube. In this case, the annexation made economic sense, as gold and silver mines in Dacia more than paid for the expenses of defending the province. The Romans also sent a clear political message to their opponents on the frontiers: “Annoy us enough and we will come and entirely destroy you and your homes.” (Six decades and three emperors later, Marcus Aurelius (r. 161–180) similarly waged systematic campaigns against the Marcomanni and Quadi in central Germany after those tribes had broken into the empire and reached northern Italy.)

Following his victories over the Dacians, Trajan waged a great campaign against the Parthians in the east and turned the Mesopotamian river valley into a province. That effort made little strategic sense, however, as the distances were simply too great. Trajan’s successor, Hadrian (r. 117–138), pulled back to the traditional border between Rome and Parthia along the eastern borders of modern-day Syria and Turkey. Again, a strategic decision rested on what made logistic and political sense.

In addition to engaging foreign enemies, the Roman army served another major strategic purpose —maintaining the empire’s internal stability. While most legions were stationed on the frontiers, some were kept close to major strategic points within the empire. Legionaries guarded Egypt, for instance, as it was a crucial source of Rome’s food supply. Other legions in Syria not only watched the borderlands between the Roman and Parthian Empires, they were ideally placed to intervene in Judea, should that province’s restless population rebel. Rebellions did flare up throughout the first two centuries of the empire’s existence, but usually only in areas that had recently been added and were least assimilated to Greco-Roman civilization.

The most dangerous rebellions arose in Judea. The first, in 66–70, prompted a campaign to re-establish Roman authority province-wide and led to the eventual destruction of Jerusalem and the temple by Vespasian (r. 69–79) and Titus (r. 79–81). The second rebellion, of Bar Kokhba in 132–136, occasioned an even harsher Roman response— a contributing factor in the great Jewish diaspora that continued through the 20th century.

The commander of the legions and auxiliaries that destroyed the Jewish rebels, General Sextus Julius Severus, was recalled from Britain to take military command in Judea (soon to be renamed “Palestine” by Hadrian). Significantly, Hadrian had had no qualms about appointing the highly competent Severus to command Roman forces in Britain at one end of the empire and then transferring him across the breadth of the empire to confront the crisis in Judea—an indication that at least during this period, the emperors could depend on their generals’ fidelity.

Augustan calculations of the force structure required for the empire’s defense remained accurate largely because the size of the Roman army remained stable for two centuries. By the reign of Claudius (r. 41–54), the army was back to a force of 28 legions, the same number Augustus had established, with three of the legions guarding the new province of Britannia, which Claudius had conquered. By the end of the reign of Septimius Severus in the early 3rd century, the army had stabilized at 33 legions. That force stability, despite a number of major wars of conquest, suggests Rome’s rulers had carefully calculated the economic burden the empire could bear in supporting its defense establishment. That affirms a grand strategy that had carefully weighed the connection between ends (the protection of the frontiers) and means (the available resources to support the army).

While the Roman world was not a pleasant place for the great majority of its people—life, in the words of the 17th century English philosopher Thomas Hobbes, was “brutal, nasty and short”—the Mediterranean enjoyed two centuries of peace absent the murderous ravages that have marked human polity through the ages and which, in fact, rippled across that same world through the 20th century.

Unfortunately for the Romans and their strategic wisdom, the world is always in flux. In the first half of the 3rd century, the external environment experienced major changes, while the empire itself underwent equally significant economic and political changes. During that time, Rome’s civil-military relations, which had remained stable for most of the previous 250 years, collapsed. Historians have accurately called this period the “crisis of the 3rd century.”

Nearly all the major developments that fed the crisis—military, civilian and economic—remain obscured by the dearth of historical sources. Yet, historians can pinpoint some specifics through the mists of the past.

During the 2nd century reign of Marcus Aurelius, soldiers returning from a campaign in the east against the Parthians apparently brought back some sort of plague. In evaluating the impact of such a development, it is important to remember how finely balanced was the empire’s economy, producing only a slight food surplus. Any significant disease-driven decline in population would translate into a smaller food surplus, in turn hindering the empire’s ability to meet its commitments. Its forces spread thinly along vast frontiers, Rome could not afford to cut military expenditures to keep the aggressive barbarians at bay.

Those frontiers grew more threatening in the 3rd century. The Parthians, rarely successful against the legions, yielded during the period to the Sassanid Empire, a far more resilient and effective military state. Consequently, Rome could expect no more easy victories in the east, but rather the constant threat of invasion into the empire’s richest provinces. In the west, by the beginning of the 3rd century, tribes beyond the frontier were fielding increasingly sophisticated bands of armed men. The barbarian tribes east of the Rhine and north of the Danube coalesced into larger and more aggressive groupings—perhaps emboldened by the internal troubles they perceived within the empire.

Still, massed barbarians should not by themselves have represented a serious threat to Rome. After all, Trajan had handled the Dacians and Parthians in two impressive wars, and Marcus Aurelius had repelled the Germanic tribes that had reached into northern Italy. Similarly, in the late 2nd century BC, Gaius Marius and Lucius Cornelius Sulla had crushed the invasion of the Teutons and the Cimbri, who had just destroyed two great consular armies. The Romans had faced and handled many such invasions in the past and, as Marcus Aurelius proved, the superiority of their military system even allowed them to absorb significant military defeats.

Changes on the domestic front, however, shifted the balance. The strains of supporting the military establishment from a shrinking economic base were clearly beginning to show, while the civil-military fabric at the highest levels of the empire unraveled in the early 3rd century. Generals and common soldiers alike began to question the emperor’s legitimacy. In short, the provincial armies, as well as the praetorians, came to feel it was their right to determine who would rule the empire.

Between Augustus’ assumption of dominion in 31 BC and the reign of Septimius Severus (AD 193–211), and excluding those emperors caught up and killed in the two civil wars, 10 rulers died of natural causes, one (Claudius) was possibly murdered by his wife, one (Nero) committed suicide and at least three others were assassinated by soldiers or officers of the praetorian guard or members of their entourages. Admittedly, there was no simple, constitutional tradition whereby a dying emperor could transfer power to his successor. But the Augustan principate had worked relatively well, sparking just two major contested wars of succession over three centuries.

In his history of the first great civil war that brought Vespasian to power in AD 70, Tacitus commented that the legions had “discovered the secret of the empire”—namely that they, and they alone, were the arbiters of who would rule in Rome. Given the history of the next 150 years, Tacitus was certainly wrong at the time, but he was right in terms of what was to happen in the 3rd century: Troubles were brewing under the reign of Caracalla, the son of Septimius Severus, who had followed his father’s advice to favor the soldiers and forget the rest. But Caracalla was a monster whom even the soldiers came to distrust. In 217, at the instigation of a praetorian prefect, an officer of the emperor’s bodyguard assassinated Caracalla just six years into his reign, setting a precedent for successions to follow.

All of Septimius Severus’ successors were assassinated by their officers or courtiers, clearly signifying a breakdown in the loyalty of senior officers. Thus, despite the decline in overall military effectiveness, militarization of the empire had accelerated; the legions were focused on promoting their own candidates to the purple rather than on defending the empire from its external enemies.

In 235 the Danubian and Rhine legions, concentrated in Gaul for a campaign against German invaders, took issue when Emperor Alexander Severus (r. 222–235) sought to placate instead of punish the tribesmen. The legions mutinied, murdering the emperor.

His successor, Maximinus Thrax (r. 235–238), a man of great size and strength, had risen from the ranks of the army. Unlike the emperors of the first two centuries—who had come almost entirely from the senatorial class, with ample knowledge and understanding of the broader interests of the empire— Maximinus had no connection to the upper classes, nor did the legitimacy of his rule rest on any other basis than the sharpness of his soldiers’ swords. In the 1st and 2nd centuries, senators had spent the bulk of their careers in handson civil positions, actually running the government. But in the 3rd century, the path to the top hinged on one’s military service—social class, education and political experience played little or no role.

The turbulent careers of Alexander Severus’ successors speak volumes about the collapse of civil discipline within the army and particularly among its senior officers. A seemingly unending series of civil wars and revolts buffeted the armies from one end of the empire to the other—conflicts that undermined the economy and civil structure. In a period of just 40 years, only one emperor died of natural causes (plague)— hardly an indication of political stability or strategic wisdom.

The strife of those years had catastrophic results. Most obvious was that the barbarians—increasingly emboldened by the scenes of Romans fighting Romans in successive civil wars and revolts—devastated substantial portions of the empire. At first, localized raids ravaged the border provinces. But barbarian thrusts grew increasingly ambitious, even reaching into Italy, which explains the city walls Emperor Aurelian (r. 270–275) hastily constructed around Rome. Raids by the Goths and the Heruli in the 250s and 260s devastated the province of Dacia and reached into the Balkans and through the Dardanelles to wreck much of Greece and western Asia Minor. At the same time, the Sassanids under King Shapur overran the border kingdoms between Persian and Roman territory and reached into Syria to capture Antioch, one of the empire’s greatest cities—along the way they destroyed a Roman army and captured Emperor Valerian (r. 253–260).

The economic toll these raids and other upheavals exacted on the empire cannot be exaggerated. The disasters touched off an economic death spiral— as the external threats demanded greater military expenditures, the invasions inflicted enormous damage on the empire’s subsistence economy.

But there was another significant problem: The Roman army of the empire’s first two centuries had depended on the extraordinary discipline and training of its legions to overcome the numerical superiority and ferocity of its enemies. Romans of that period won battles and wars because they were pitting disciplined, highly trained professionals against the equivalent of armed mobs. But the revolts and assassinations of the 3rd century, unlike the civil wars of the late republic, were a byproduct of collapsing military discipline among officers and soldiers. Absent such discipline and training, military professionalism fell by the wayside. This wasn’t a sudden event. As the declining economy led to the gradual constriction of the army, the disintegration of discipline led to an army that increasingly resembled the armed mobs of its opponents. The Western Roman Empire had reached a tipping point. Its collapse was now inevitable.

For further reading, Williamson Murray recommends: The Grand Strategy of the Roman Empire, by Edward Luttwak, and The Cambridge Ancient History, Vols. 9 and 10.

Originally published in the November 2010 issue of Military History. To subscribe, click here.

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